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For a transaction to be isolated, the changes that are part of the transaction should not be available until the transaction is complete. For the bank account example, you wouldn t want a credit check to view your bank account totals when only the withdrawal had occurred. In the real world, different levels of transaction isolation are available because the computational cost of creating completely isolated transactions can become very expensive. However, even in these situations, it is incumbent on the development team to be aware of the possibility that the information being requested might be in a state of transition.
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A transaction that is durable must survive failure: After the transaction has been committed, any kind of failure that might affect the data store (power outage, hardware failure, and so on) must not impact the data that was updated as part of the transaction. When the transaction is run against a single data store, ensuring durability is relatively straightforward. However, when there are multiple data stores on multiple computers, the complexity of ensuring durability is more challenging. This chapter addresses those challenges.
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Of the four fundamental properties of a transaction, which one cannot be directly addressed by a resource manager such as a database Quick Check Answer The integrity of the transaction is, to a certain extent, up to the developer to ensure. Although databases are capable of evaluating certain types of integrity constraints (referential, for example), they cannot evaluate a transaction s integrity from the perspective of business logic. Consider a transaction that involves transferring money from one account to another. The inclusion of both a debit and a credit update within the transaction, as well as ensuring that the amounts match, is something that only the developer can guarantee.
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Implementing ACID
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For a transaction to meet the ACID test successfully, a number of techniques can be used. Even against a single data store, a transaction can require many small operations to be performed, including updating indices, reorganizing rows, or executing triggers. When the transaction is distributed across different data stores running on difference physical machines, the problems increase greatly.
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Lesson 1: Transaction Basics
In a single data store, there are two common approaches to implementing ACID. Record locking ensures that multiple simultaneous attempts to read or update information cannot occur. Durability is ensured through write-ahead logging (writing updates to a log file prior to updating the data) or shadow paging so that pages are copied when they are written to, and the original page is updated only when the transaction is committed. In a distributed environment, the most common approach is to use a two-phase commit approach that allows all the nodes in the transaction to vote on success or failure before the commit occurs. As its name suggests, the two-phase commit has two distinct phases.
Prepare Phase
The transaction coordinator coordinates the phases. The transaction coordinator is typically located on the computer that initiates the transaction. After all the updates have been performed, a request is made to the transaction coordinator to commit the transaction. The transaction coordinator sends a Prepare request to a transaction manager on each participating computer. Each transaction manager is responsible for determining whether the updates within the scope of the resources that it manages are valid. If the transaction is valid, the transaction manager returns a Success message to the transaction coordinator. If there is a problem with the updates, the transaction manager returns a Failure message to the transaction coordinator. After all the transaction managers return either a Success or a Failure message, the Prepare phase is complete.
Commit Phase
The second phase in the process has two possible paths. If all the transaction managers return a Success message, the transaction coordinator sends out a Commit request to all the transaction managers. Upon receipt of this message, the transaction manager performs a commit on the updates and returns an acknowledgement to the coordinator. When the coordinator has received all the acknowledgements, the transaction is considered complete. The second possible path occurs if any of the transaction managers returned a Failure message in the Prepare phase. A single failure means that all the updates should be rolled back, so the transaction coordinator sends out an Abort request to the transaction managers. The managers perform a rollback of the updates under their control and send an acknowledgement to the coordinator. This is the basic flow through the two-phase commit process. Naturally, there are a number of other options. For example, the Prepare phase will have a timeout so that if no response is received from one of the transaction managers after a period of time, the transaction will be aborted. The same would happen if a transaction manager didn t receive a commit or abort message within a reasonable time frame. Also, from a responsiveness perspective, the entire
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